U.S. patent application number 10/847044 was filed with the patent office on 2004-10-28 for non-contact technique to monitor surface stress.
Invention is credited to Bamberger, Robert J. JR., Frazer, R. Kelly, Grossman, Kenneth R., Miragliotta, Joseph A..
Application Number | 20040211264 10/847044 |
Document ID | / |
Family ID | 26848810 |
Filed Date | 2004-10-28 |
United States Patent
Application |
20040211264 |
Kind Code |
A1 |
Miragliotta, Joseph A. ; et
al. |
October 28, 2004 |
Non-contact technique to monitor surface stress
Abstract
A non-contact method for evaluating stress in a substrate. An
impurity is non-uniformly introduced into at least one region of a
crystalline substrate. The crystalline substrate is subjected to
physical stress. Fluorescence producing energy is directed at the
crystalline substrate. A fluorescence produced by the crystalline
substrate is measured. The fluorescence is correlated with the
stress on the crystalline substrate.
Inventors: |
Miragliotta, Joseph A.;
(Ellicott City, MD) ; Grossman, Kenneth R.;
(Olney, MD) ; Frazer, R. Kelly; (Highland, MD)
; Bamberger, Robert J. JR.; (Baltimore, MD) |
Correspondence
Address: |
Office of Patent Counsel
The Johns Hopkins University
Applied Physics Laboratory
11100 Johns Hopkins Road
Laurel
MD
20723-6099
US
|
Family ID: |
26848810 |
Appl. No.: |
10/847044 |
Filed: |
May 17, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10847044 |
May 17, 2004 |
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10151629 |
May 20, 2002 |
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6763727 |
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60292254 |
May 18, 2001 |
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Current U.S.
Class: |
73/800 |
Current CPC
Class: |
G01L 5/0047 20130101;
G01L 1/24 20130101 |
Class at
Publication: |
073/800 |
International
Class: |
H01L 021/301 |
Goverment Interests
[0002] This invention was made with Government support under Naval
Sea Systems Command contract no. N00024-98-D-8124, Arlington, Va.
The Government has certain rights in the invention.
Claims
We claim:
1. A device for non-contact evaluation of stress in a substrate,
the device comprising: a hollow cylindrical window support operable
to support the substrate; a source of fluorescence producing energy
operable to direct the fluorescence producing energy at the
substrate; a heat source operable to subject the substrate to
elevated temperature; a mechanical loading assembly operable to
subject the substrate to a mechanical load; and a sensor operable
to detect fluorescence emitted from the substrate.
2. The device according to claim 1, wherein the mechanical loading
assembly comprises a substrate contacting surface.
3. The device according to claim 1, wherein the mechanical loading
assembly comprises a shaped force application member operable to
contact a surface of the substrate.
4. The device according to claim 1, wherein the device applies
tensile and compressive forces to the substrate.
5. The device according to claim 1, wherein the supporting cylinder
has a circular cross-section having a diameter small than a smaller
of a length and a width of the substrate.
6. The device according to claim 5, wherein the load assembly
applies the mechanical load to the substrate centered with respect
to the cylinder.
7. The device according to claim 1, further comprising: a cooling
assembly for cooling at least the source of fluorescence producing
energy and the sensor.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a divisional application of application
Ser. No. 10/151,629, filed May 20, 2002, which claims the benefit
of U.S. provisional application No. 60/292,254, filed on May 18,
2001, each of which is hereby incorporated by reference in its
entirety.
BACKGROUND OF THE INVENTION
[0003] 1. Field of the Invention
[0004] The present invention relates to evaluating stress on a
surface without contacting the surface.
[0005] 2. Description of the Related Art
[0006] Many advanced defense missile systems use an infrared (IR)
seeker for the purpose of identifying and tracking the intended
target of interest. Due to the nature of the aerothermal flight
environment, the protective IR transparent window must be able to
survive extremely high thermal stresses (>100 MPs) in order to
prevent catastrophic failure. In many missile systems, the window
material of choice has been crystalline sapphire, which has both
optical and mechanical properties that are suitable over a wide
range of operational flight conditions.
[0007] To assure the safe operation of a seeker window, the
performance of the IR window under realistic stress and temperature
conditions typically is examined. In this examination, the
threshold limits of the material making up the window may be
determined. Since the conditions encountered in use typically are
extreme, the same conditions typically are encountered in testing.
Testing often occurs in a wind tunnel. However, the measurement of
sapphire window strain in hypersonic wind-tunnel applications is
very difficult. Aerothermal heating and shear usually preclude the
mounting of common strain gauges on the front side of windows under
test. Back-side mounting is complicated by the extreme temperatures
commonly seen by these windows.
[0008] In many test simulations, sample temperatures can easily
exceed 500 degrees C. and can extend to 1000 degrees C.
Temperatures of this magnitude prohibit the use of conventional,
direct-contact strain gauge transducers. Along these lines, strain
gauge adhesives typically break down at temperatures in the
vicinity of 320 degrees C.
[0009] Mounting strain gauges on the back-side of the windows also
does not allow the measurement of front-surface stresses. The
physical size of strain gauges reduces spatial resolution and does
not allow for a high density of measurements. Strain gauges are
intrusive and can affect thermal gradients and, thereby, local
strain gradients on the material under test. Crystalline windows
are also commonly used with corrosives where strain gauges are
attacked by the surrounding media.
[0010] Optical fluorescence provides an alternative approach to
direct contact probing. Optical fluorescence relies on the ability
to generate emission from ions such as chromium, magnesium, and
vanadium that are embedded in a crystalline lattice of window
materials. For example, it is known that chromium ions in
crystalline sapphire produce a narrow-band fluorescence doublet in
the red region of the spectrum. The doublet is sensitive to both
temperature and stress in the sample. These two intense emission
lines are termed the R-fluorescence lines.
[0011] The effect of an applied stress to a sapphire window is the
distortion of the crystal field surrounding the chromium ion. The
distortion changes the potential energy of the ion and, hence, the
emission wavelength of the fluorescence radiation. Thus, the effect
of stress can be quantitatively calibrated as a shift in the
characteristics of the R-fluorescence lines and used as a
non-contact probe of stress in sapphire windows.
SUMMARY OF THE INVENTION
[0012] The present invention provides a non-contact method for
evaluating stress in a substrate. The method includes non-uniformly
introducing at least one impurity into the crystalline substrate.
The crystalline substrate is subjected to physical stress.
Fluorescence producing energy is directed at the crystalline
substrate. A fluorescence produced by the crystalline substrate is
measured. The fluorescence spectrum is correlated with the stress
on the crystalline substrate.
[0013] The present invention also includes a method for
manufacturing a structure for non-contact evaluation of stress in
the structure. According to the method at least one impurity is
non-uniformly introduced into a crystalline substrate.
[0014] Additionally, the present invention provides a structure for
non-contact evaluation of stress in the structure. The structure
includes a crystalline substrate including at least one impurity
non-uniformly distributed in the substrate.
[0015] Furthermore, the present invention provides a device for
non-contact evaluation of stress in a substrate. The device
includes a hollow cylindrical window support operable to support
the substrate. A source of fluorescence producing energy is
operable to direct the fluorescence producing energy at the
substrate. A heat source is operable to subject the substrate to
elevated temperature. A mechanical loading assembly is operable to
subject the substrate to a mechanical load. A sensor is operable to
detect fluorescence emitted from the substrate.
[0016] Still further, the present invention provides a non-contact
method for evaluating stress in a sapphire window. The method
includes subjecting to a physical stress a sapphire window that
includes at least one impurity non-uniformly distributed in at
least one region in the vicinity of at least one surface of the
sapphire window. Fluorescence producing energy is directed at the
sapphire window. A fluorescence produced by the sapphire window is
measured. The fluorescence spectrum is correlated with the stress
on the sapphire window.
[0017] Still other objects and advantages of the present invention
will become readily apparent by those skilled in the art from a
review of the following detailed description. The detailed
description shows and describes preferred embodiments of the
present invention, simply by way of illustration of the best mode
contemplated of carrying out the invention. As will be realized,
the present invention is capable of other and different embodiments
and its several details are capable of modifications in various
obvious respects, without departing from the present invention.
Accordingly, the drawings and description are illustrative in
nature and not restrictive.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] Objects and advantages of the present invention will be more
clearly understood from the following specification when considered
in conjunction with the accompanying drawings, in which:
[0019] FIG. 1 represents a graph that illustrates a relationship
between depth below the surface of a sapphire window and chromium
concentration for an embodiment of a chromium doped sapphire window
according to the present invention;
[0020] FIG. 2 represents a cross-sectional view of an embodiment of
a calibration device according to the present invention;
[0021] FIG. 3 represents a cross-sectional view of a portion of an
embodiment of an apparatus according to the present invention
carrying out calibration on an embodiment of a sapphire window;
[0022] FIG. 4 represents a graph that illustrates relationships
between emission wavelength and emission intensity when subjecting
a sapphire window to various temperatures;
[0023] FIG. 5 represents a graph that illustrates relationships
between emission wavelength and emission intensity for an
embodiment of a sapphire window according to the present invention
and a known sapphire window;
[0024] FIG. 6 represents a graph that illustrates a relationship
between temperature and peak position of fluorescence bands
produced by an embodiment of a sapphire window according to the
present invention;
[0025] FIG. 7 represents a graph that illustrates relationships
between fluorescence wavelength and fluorescence intensity produced
by an embodiment of a sapphire window according to the present
invention when subjected to various types of loads;
[0026] FIG. 8 represents a cross-sectional view of a sapphire
window illustrating a force being applied thereto;
[0027] FIG. 9 represents a cross-sectional view of an embodiment of
a sapphire window according to the present invention; and
[0028] FIG. 10 represents a cross-sectional view of another
embodiment of a sapphire window according to the present
invention.
DETAILED DESCRIPTION
[0029] The present invention provides a method, device, and
structure that permit non-contact measurement of stress on the
structure. The stress is measured through optical techniques,
measuring fluorescence produced by one or more materials introduced
into the structure. The structure typically includes a crystalline
substrate with one or more materials introduced non-uniformly in
the crystal structure.
[0030] According to one particular application, the present
invention provides an optical technique for measuring surface
stress in chromium-doped sapphire windows. The approach utilizes
the known effects of temperature and stress on the spectral profile
of chromium ion fluorescence in crystalline sapphire. According to
the present invention, sapphire windows may be selectively doped
with a surface concentration of chromium ions, which provides a
direct measure of the stress and temperature in the surface region
of the window. A series of fluorescence measurements may be
performed in a calibration apparatus to calibrate the effects of
temperature and mechanical stress on the spectral characteristics
of the surface fluorescence. The fluorescence can later be
correlated with the spectra measured during substrate use or
representative testing to determine in-situ stress. The present
invention is particularly useful as a dynamic, non-contact probe of
stress in infrared seeker windows while under simulated conditions
of flight.
[0031] Although the present invention is discussed with respect to
sapphire, it may be employed with other crystalline materials. For
example, the invention may be employed with a yttria crystalline
body. Other crystalline materials that the present invention may be
employed with include silica and organic salts.
[0032] Similarly, although the present invention is discussed with
respect to chromium as a chromophore, other chromophores may also
be employed. For example, neodymium could be employed. Other
chromophores that could be utilized include rare earth ions similar
to neodymium. One example of such an ion is erbium.
[0033] In practice, any combination of dopants and/or crystalline
substrates could be utilized that would permit monitoring of stress
on the substrates. Dopant selection is largely dependent upon the
substrate being monitored.
[0034] According to the present invention one or more impurity is
distributed in at least one region in a substrate. Often, the
impurity(ies) is selectively confined to one or more regions in the
vicinity of at least one surface of the substrate. For example, the
impurity could be distributed in a single region that extends
substantially entirely along one surface of a substrate (see FIG.
9). According to another embodiment, the impurity is distributed to
a plurality of regions in the vicinity of opposite surfaces of a
substrate (see FIG. 10). The chromium or other impurity may be
non-uniformly distributed with respect to depth. Any number of
regions on any combination of surfaces may be employed if the
arrangement permits monitoring stress as described herein.
[0035] In an embodiment where one or more impurities is/are
introduced into one or more regions in the vicinity of one or more
surfaces of a substrate, the regions may have a maximum depth of
about 10 nm to about 200 nm. The peak concentration of the
impurity(ies) is typically about 10.sup.20 to about 10.sup.22
ions/cm.sup.3.
[0036] During use, crystalline substrates are often stressed
differently at each surface. For example, optical windows are often
in compressive stress on one surface while in tensile stress on the
other. Impurities distributed in the substrate bulk typically would
not result in a useful spectral profile since the varied
contributions of impurities at various positions throughout the
substrate typically could not be deconvoluted.
[0037] Selectively embedding the impurity(ies) in one or more
regions in the vicinity of the surface of a substrate allows for
measurement of local stress at that surface. As described in
greater detail below, confining the impurity(ies) to region(s) in
the vicinity of a surface can result in a spectral profile of
surface fluorescence that is sensitive to both applied temperature
and mechanical loading on that substrate surface. As a result, the
optical signature may be utilized to quantify the amount of stress
at the surface of the substrate. The distribution of impurity(ies)
according to the present invention can influence the ion
fluorescence in a way that is contrary to previously reported
results obtained when utilizing a uniformly doped sapphire
substrate.
[0038] Typically, the substrate employed according to the present
invention is crystalline. However, the substrate could also be
amorphous. If the substrate is crystalline, it may be
monocrystalline. The crystalline substrate may be sapphire or
another material.
[0039] The structural uses of single-crystal sapphire commonly
include use as a window material in applications with high
pressures, high temperatures, and aggressive chemicals. Sapphire
windows are very strong and very hard. The transparency of the
windows in the near IR makes them suitable as seeker windows on
high-speed missiles. They are also highly resistant to chemical
attack even at high temperature, making them very desirable windows
for applications where acids and alkali are in use, particularly at
high temperature. The crystalline and electrical properties of
sapphire also make it desirable as a semiconductor substrate.
[0040] A variety of techniques may be employed to produce the doped
region(s). One embodiment of the present invention employs a
high-energy ion implantation technique for post-growth doping. This
technique can create a highly non-uniform chromium distribution in
the vicinity of one or more surface regions of a substrate, such as
a sapphire window. According to one embodiment that utilizes a
high-energy ion implantation technique, the implant is carried out
at a temperature of about 1000 degrees C. with a beam energy of
about 150 keV and an ion flux of about 10.sup.17 per square
centimeter. Such parameters produce the concentration profile of
chromium shown in FIG. 1, which resulted from a transport of ion in
matter (TRIM) calculation. Alternatively, introduction of
impurities into the substrate can be accomplished through other
common microelectronic fabrication techniques such as diffusion,
chemical vapor deposition (CVD), liquid phase epitaxy (LPE), or
molecular beam epitaxy (MBE). Diffusion allows impurity deposition
to several microns of the substrate surface. CVD and epitaxy
techniques allow impurity deposition at any desired position within
the substrate.
[0041] The sapphire substrates utilized in the examples described
herein had a diameter of about 25.4 mm, a thickness of about 1 mm,
and a crystalline c-axis oriented normal to the surface of the
window. Such a sapphire substrate may be obtained from Crystal
Systems, Inc., of Salem, Mass. The sapphire substrates received
from the manufacturer were probed via optical fluorescence and
observed to have no detectable level of chromium impurity. The
sapphire substrates were subsequently doped with a chromium ion
concentration procured from Epion Corporation of Billerica, Mass.,
using the technique of high-energy ion implantation described
herein.
[0042] Masks may be employed in the implantation to develop surface
patterns. This can permit increased isolation between measurement
centers. Front- and back-surface stress measurements can also be
taken simultaneously on one sample by offsetting doping sites, as
illustrated in FIG. 10. Along these lines, FIG. 10 illustrates a
high-purity sapphire substrate being doped with chromium ions
utilizing mask structures 25 and 27 on both sides of the substrate
to confine the chromium ions to selected locations. On the other
hand, FIG. 9 illustrates chromium being doped along a single
surface of a sapphire substrate.
[0043] Typically, no post implantation processes, such as annealing
are utilized. However, annealing may be carried out after
implantation to tailor the distribution of the ion concentration in
the substrate.
[0044] The dopant is introduced into the region in the vicinity of
the surface of a substrate to a maximum concentration of about 10
nm to about 200 nm. As described above, the concentration varies by
depth. According to one embodiment, chromium ion is introduced into
a sapphire substrate such that the peak concentration of chromium
ion is about 10.sup.22 ions per cubic centimeter at a depth of
about 70 nm below the surface of the sapphire. This concentration
is significantly higher than the approximate 10.sup.17 cm.sup.-3
bulk concentration level observed in most commercially grown
sapphire windows.
[0045] To determine its ability to withstand stresses that it could
encounter in use, a substrate is subjected to stress. As described
above, a sapphire window reacts in a way that alters its
fluorescence, thereby making it possible to determine a window's
capacity to withstand stress. Any stress that a substrate may
encounter in use may be applied to the substrate in a test setting
to determine how the substrate reacts to stress.
[0046] In the testing setting, as any stress is applied to a
substrate exciting energy is directed at the window. The exciting
energy has characteristics that cause the chromium or other
impurity in the substrate to fluoresce. The wavelength of exciting
energy may depend upon the substrate and/or the impurity involved.
A chromium doped sapphire window typically is subjected to
electromagnetic radiation having wavelengths in green and/or
ultraviolet regions of the spectrum. Typically, the radiation has
wavelengths of about 450 nm to about 550 nm and an energy level of
about 2.76 to about 2.25 eV, respectively. According to one
particular embodiment, the electromagnetic radiation has a
wavelength of about 532 nm and an energy of about 10 mW. One of
ordinary skill in the art would be able to determine appropriate
characteristics of exciting radiation to direct at a substrate
suitable for the substrate, impurity(ies), fluorescence, stress,
and/or other parameters.
[0047] The fluorescence produced can vary with the substrate and/or
the impurity(ies) involved. A chromium doped sapphire window
typically produces fluorescence having wavelengths of about 670 to
about 720. Typically, fluorescence in a band having a wavelength of
about 690 to about 700 nm is most useful. The characteristics of
the fluorescence are measured and analyzed to correlate the
fluorescence produced with the stress being applied. Typically, the
peak position, bandwidth, and intensity are most useful in the
analysis.
[0048] When a calibration of a substrate is being performed, the
type and magnitude of stress applied to a substrate typically
depends upon the stress that would be encountered in an application
of interest. Along these lines, if the application is as a window
for a targeting device as described above, the stresses encountered
in such an application would be applied to the window during
calibration. Exemplary embodiments include an application of
mechanical force to a substrate of about 100 to about 1000 MPa. The
stress may be applied to one or more locations on a substrate. The
same or a different force may be applied to the substrate at each
location. According to one embodiment, force is applied to one
point or region centrally located on the substrate.
[0049] The fluorescence spectra is affected by substrate
temperature as well as by stress. Therefore, the substrate must be
calibrated at temperatures representative of those encountered in
the application of interest. In calibration, a substrate may
additionally or alternatively be subjected to temperatures above
room temperature. Temperatures typically are about 22 degrees C. to
about 600 degrees C.
[0050] Any apparatus that can apply heat, pressure and other
stresses to a substrate while directing exciting energy toward the
window and monitoring fluorescence produced may be employed to
calibrate and/or test substrates. The present invention provides a
device for carrying out non-contact techniques typically to
calibrate surface stress in substrates. FIG. 2 illustrates a
cross-sectional view of an embodiment of an apparatus that may be
used to apply stress and temperature, while exciting and collecting
fluorescence. With the embodiment shown in FIG. 2, both the optical
and mechanical loading assembly may be placed within a high
temperature heater, which can provide isothermal heating to a
substrate.
[0051] The apparatus 1 shown in FIG. 2 includes a support 3
operable to support the substrate being tested. The support may be
solid. In the embodiment shown in FIG. 2, the support includes a
hollow cylinder. The interior of the cylinder may house other
elements of the apparatus as described below.
[0052] The support may have any cross-sectional shape. Typically,
the support includes a cross-section having a similar shape to the
shape of the window being tested. Typically, the substrate being
tested and the support both have a circular shape.
[0053] The support may include stabilizing members extending from
laterally from the support. The stabilizing members help to ensure
that the support remains in place as stress is force is applied to
the substrate. Two stabilizing members 5 are visible in the view of
the embodiment shown in FIG. 2.
[0054] The support may be made of any suitable material. The
embodiment shown in FIG. 2 includes a stainless steel support.
Other materials such as graphite could also be utilized.
[0055] The sapphire window 7 is arranged on the support 3. A load
is applied to the window. The load may be supported by a load
support 9 through a supporting arm 11. The load support and
supporting arm may have any configuration, including shape and
size. The embodiment shown in FIG. 2 includes a stainless steel
load supporting arm. Other materials such as graphite could also be
utilized.
[0056] A load applying surface contacts the surface of the
substrate to apply the load. The load applying surface may be the
tip of the supporting arm or load support or even the load itself.
The load application surface may have any desired contour. Along
these lines, the load application surface could be planar.
Alternatively, the load application surface could have a spherical
contour, could terminate in a point, or have any other contour.
[0057] The load may also be applied to the substrate with a
separate load application member. For example, the embodiment shown
in FIG. 2 includes a load application member 13. The load
application member includes the load application surface have any
contour as described above.
[0058] FIG. 3 illustrates a close-up cross-sectional view of a
portion of the apparatus shown in FIG. 2. FIG. 3 illustrates the
upper portion of the substrate supporting member 3. The supporting
member has a diameter of about 0.674 inches. The diameter of the
supporting member 3 is smaller than the diameter of the substrate,
which is in the shape of a disc. The substrate has a thickness t. A
load W is applied to the substrate with ball 13. The load typically
includes calibrated weights that apply a known mechanical stress. A
ring may be used and is often preferred to a ball. The diameter of
such a ring should be very small compared to the diameter of the
window. In the embodiment shown in FIG. 3, the ball has a diameter
of about 4 mm. The ball, ring or other load applicator may be made
of any suitable material. Typically, the hardness of the material
will permit application of a desired load. Examples of materials
that may be utilized include stainless steel, as in the embodiment
shown in FIG. 3, ceramic, carbon, or the materials could also be
employed. Other materials that may be employed that can withstand
the high load and term.
[0059] Employing a spherical load application surface is a good
configuration to permit application of a compressive and tensile
stress in the top and bottom surfaces, respectively. The assembly
shown in FIG. 3 is designed to apply the load as a point contact
onto the top surface the substrate, located concentrically with
respect to the bottom graphite cylinder ring.
[0060] In the embodiment shown in FIGS. 2 and 3, the upper ring may
not be rigidly attached to the upper graphite rod. The ring may be
flush with the sample to help transfer a well distributed load
properly into the sample. A smooth cup-and-ball socket may be used
to allow the upper ring to align with the sample.
[0061] An apparatus for calibrating and/or testing a substrate also
includes a source of exciting energy that can produce fluorescence
in the substrate. The parameters of the energy are discussed above
in greater detail. Any suitable energy source may be employed. For
example, the embodiment shown in FIG. 2 includes an optical fiber
assembly to deliver the energy to the substrate. The system shown
in FIG. 3 delivers green light to a sapphire window substrate.
According to one particular embodiment, a double Nd:YAG laser
producing radiation having a wavelength of about 532 nm and
delivered to the substrate through a single optical fiber. The
infrared fluorescence having a wavelength of about 695 nm to about
750 nm is collected by a single optical fiber according to this
embodiment.
[0062] An apparatus for calibrating and/or stress testing
substrates also includes at least one element to sense, detect, or
otherwise collect the fluorescence produced by the substrate as a
result of interacting with the exciting energy. The embodiment
shown in FIG. 2 includes a fiber optic assembly 21 to collect the
energy produced by the substrate as it fluoresces. The fluorescence
may be collected by one or more optical fibers positioned in the
vicinity of a substrate. Typically, optical fibers for collecting
the fluorescence are positioned as close as possible without
touching. According to one embodiment, the optical fibers are
positioned about 1 mm to about 3 mm away from a substrate.
[0063] The optical fibers collect the fluorescence and deliver it
to a device that can analyze it. For example, although not shown in
FIG. 2, the fluorescence from the optical fiber may be transmitted
to a high spectral resolution spectrometer and CCD array detector
for analysis. The emitted fluorescence is collected using a closely
positioned optical fiber and routed to a narrow-band spectrometer.
Changes in wave number, magnitude, and half-width of the emission
lines may be evaluated to extract stress information.
[0064] The source of exciting energy and the sensing or collecting
elements in an apparatus for testing and/or calibrating a substrate
may be arranged anywhere as long as the energy can be delivered and
fluorescence sensed or collected. In the embodiment shown in FIG.
2, these elements are arranged are arranged in the window support.
According to the embodiment shown in FIG. 2, the exciting and
collecting assembly was placed within a few millimeters of the
substrate, providing both the source for optical excitation and
collection of the emitted fluorescence signal.
[0065] Depending upon their composition, the source of exciting
energy and the sensing or collecting elements in an apparatus for
testing and/or calibration may be sensitive to heat applied to a
substrate. For example, the relatively low threshold temperatures
of an optic fiber system may make the cooling apparatus a
necessity. As a result, the source of exciting energy and the
sensing or collecting elements may require cooling.
[0066] To accomplish this cooling, an apparatus according to the
present invention may include a cooling assembly. For example, the
embodiment shown in FIG. 2 includes a water cooling system 15 to
cool the source of exciting energy and the sensing or collecting
elements. The cooling system 15 includes a jacket that surrounds
the source of exciting energy and the sensing or collecting
elements and bathes them in water. The cooling water may also be
present within the supporting cylinder 3.
[0067] The cooling assembly shown in FIG. 2 shrouds the assembly
within a water-cooled copper tubing package. The shroud can
maintain a moderate temperature, such as on the order of less than
about 50 degrees C., near the tip of each fiber when maximum
heating conditions were applied to the sample, such as on the order
of about 500 degrees C. Any coolant may be utilized to cool
portions of the assembly.
[0068] An apparatus for calibrating and/or testing a substrate may
also include a heat source and/or cooling source to subject a
substrate to temperatures above and/or below room temperature. When
calibrating and/or testing a substrate, the temperature exposure
may be non-uniform. The embodiment of the apparatus shown in FIG. 2
includes a heater 17 to subject the substrate to heat to simulate
conditions that the substrate may encounter in an end use. Any heat
source may be utilized with the present invention. To maintain the
heated environment, at least the portion of the apparatus that
includes the substrate may be enclosed within an insulating
enclosure 19. In the embodiment shown in FIG. 2, portions of the
substrate support 3 and the load support 11 are enclosed within the
insulating enclosure.
[0069] As described above, chromium may be utilized to dope the
sapphire window. In a sapphire lattice, chromium is known to be a
substitutional replacement for aluminum at low concentrations,
where the position of the ion is the octahedral coordinated
location adjacent to both end oxygen and aluminum ion. The
octahedron lattice of sapphire is slightly distorted (anisotropic
crystal field), which produces a doublet rather than singlet
fluorescence band profile. The doublet is termed the
"R-fluorescence" band, composed of the R.sub.1 (694.85 nm) and
R.sub.2 (693.37 nm) emission lines.
[0070] FIG. 4 illustrates the effects of temperature on the
emission characteristics of the R.sub.1 and R.sub.2 lines of the
chromium ion distribution in sapphire windows according to the
present invention. Along these lines, FIG. 4 illustrates R.sub.1
and R.sub.2 fluorescence bands from chromium doped sapphire as a
function of sample temperature 30 degrees C., 210 degrees C., 330
degrees C., 430 degrees C., and 500 degrees C. The effects of
increasing temperature are the reduction, broadening, red shifting,
and merging of the fluorescence bands, results that are analogous
to previous temperature studies of low concentration,
chromium-doped sapphire. An increase in substrate temperature may
result in a stronger interaction between the chromium ion and the
surrounding lattice, which may reduce the radioactive, i.e.,
fluorescence, efficiency. Similarly, the shift to longer
wavelengths may be a result of the internal compressive strain that
accompanies the thermal expansion of the lattice. The two emission
peaks do not appear to have the same temperature dependence, which
results in the merging of the doublet at about 430 degrees C.
[0071] The peak position and line width of the two emission bands
in the room temperature spectrum was shifted about 0.37 nm to
longer wavelengths as compared to the corresponding peaks in a
sapphire sample with a considerably lower chromium concentration,
as shown in FIG. 5. Along these lines, FIG. 5 illustrates a
comparison of chromium fluorescence bands from low concentration,
uniformly doped sample (solid line) and high peak concentration,
non-uniformly doped sample (dashed line). The emission intensity
from the non-uniform sample was approximately 75 percent lower than
that of the bulk sample.
[0072] In addition to the red shift in peak position, the line
width of the implanted sample was about 36 percent broader than the
low concentration sample. The shift in peak position may be
attributed to the volumetric strain that is induced and sensed by
the chromium ion when the ion is substituted for the smaller
aluminum ion in the sapphire lattice, similar to the internal
strain that is arises from thermal expansion of the lattice. The
effect of the larger ion within the crystal is a distortion of the
lattice, which may increase internal strain and shifts the
R-fluorescence peaks to longer wavelengths.
[0073] As was illustrated by Ma et al., Optical fluorescence from
chromium ions in sapphire: A probe of the image stress, Acta
Metall. Mater. 41, pp. 1811-1816 (1993), and Kaplyanskii et al.,
Sov. Phys. Solid St., 10, 1864 (1969), the entire contents of the
disclosure of which is hereby incorporated by reference, the
relationship between the red shift in peak position of the R.sub.1,
and R.sub.2 lines and chromium concentration is given by Equation
(1) below:
.DELTA.v=99c.sub.m (1)
[0074] where .DELTA.v represents a peak position in wavenumbers and
c.sub.m represents the chromium concentration in weight percent.
Accordingly, it is also expected that the linewidth for the
ion-implanted sample will be significantly broadened with respect
to the uniformly doped sapphire window. As shown in FIG. 1, the ion
concentration is markedly inhomogeneous, which should produce a
continuous variation in the internal stress and, hence, the peak
position of the fluorescence bands. Therefore, the results shown in
FIG. 5 indicate that there is a high peak value of internal strain
in surface layer, where the strain varies continuously throughout
the ion distribution in the surface layer.
[0075] FIG. 6 shows a plot of the peak position of the two
fluorescence bands shown in FIG. 4 versus sample temperature. The
two bands did not display the same dependence on temperature, which
led to their subsequent merging at a temperature of approximately
430 degrees C. The peak positional shifts exhibited linear behavior
over the temperature range of about 100 to about 380 degrees C.,
with a slope value of about 0.0066 nm/.degree. C. and 0.0081
nm/.degree. C. for the R.sub.1 (diamond) and R.sub.2 (triangle)
bands, respectively. Despite the non-uniform distribution of the
chromium ion concentration, it has been determined that the
temperature dependence of the fluorescence is analogous to the
corresponding emission peaks in low concentration chromium-doped
sapphire.
[0076] FIG. 7 illustrates the results of a comparison of the
fluorescence from the unstressed or unloaded sapphire window (solid
line) to a window under compressive (dot-dash line) and tensile
stress (dashed line). For the stressed measurements, a load of 50
pounds was placed on the ball-on-ring assembly shown in FIGS. 2 and
3. The observed magnitude of peak position shift is the same for
the two loading condition, 0.19 nm, but is directed towards the red
for compressive and towards the blue for tensile. There is a
shoulder in the compressive spectrum. This may result from a
different distribution of applied stress for the two measurements,
since only the compressively loaded surface is in direct contact
with the stainless steel ball. However, the qualitative behavior
illustrated in FIG. 7, that is, red-shift for compressive and
blue-shift for tensile load, is consistent with other stress
measurements of chromium doped sapphire.
[0077] The process of calibrating the effects of stress on the
spectral profile of the chromium fluorescence typically includes
calculating stress levels in a sapphire window when using a
ball-on-ring assembly such as is shown in FIGS. 2 and 3. FIG. 8
illustrates the response of a circular sample to a uniform load Q
that is distributed over a small radius r.sub.0 within a ring of
radius a. The edge of the sample is simply supported. The degree of
sample deflection is greatly exaggerated.
[0078] Under the loading arrangement shown in FIG. 8, the upper and
lower surfaces of the sample are in compressive and tensile stress,
respectively. Using the stress analysis of Roark and Young,
Formulas for stress and strain, 5th ed. (1980), the entire contents
of the 1 r = 3 W 8 t 2 [ 4 ( 1 + ) ln a r + ( 1 - ) ( a 2 - r 2 a 2
) r o ' 2 r 2 ] ( 2 )
[0079] disclosure of which are hereby incorporated by reference,
for a load applied at the center of a ring structure, the following
equations relate the magnitude of the radial stress at the surface
to the applied load on the sapphire sample. In Equation (2)
[0080] where .epsilon.r.sub.r=radial stress in the surface layer,
W=applied load, t=thickness of the sapphire window, a=radius of the
support ring, which is 0.674 in the apparatus shown in FIGS. 2 and
3, r.sub.0=radius of contact area of load with substrate,
r.sub.0'=adjusted radius of contact area of load with substrate,
r=point at which stress is being measured, and .mu.=Poisson's
ratio, which is 0.25 for sapphire. For the case in which r.sub.0'
is very small compared to the thickness of the material t,
r.sub.0', is given by equation (3) below:
r.sub.o'={square root}{square root over
(1.6r.sub.o.sup.2+t.sup.2)}-0.675t (3)
[0081] In the case of a stainless steel ball on the sapphire
sample, the radius of the contact area was considered to be
infinitesimal since the deformation associated with the applied
load in FIG. 7 was very small, (less than about 1 micron). Also,
with this ball-on-ring configuration, the maximum tangential and
maximum radial stresses are equal and are represented by
.sigma..sub.max. Since both sapphire and the metal ball are very
hard, r.sub.0 is assumed to be zero and, therefore, r.sub.0' is
assumed to equal 0.325t. For the special case in which the 2 r = 3
W 2 t 2 [ ( 1 + ) ln a r o ' + 1 ] ( 4 )
[0082] excitation source was located directly under the center of
the ring, the magnitude of the radial stress that perturbed the
chromium fluorescence is given by the equation (4) below:
[0083] Using equation (4), the maximum stress at the center of the
top and bottom surfaces of the sapphire wafer was calculated for a
50 pound load applied to the center of the ring. A value of 0.25
was used for -.mu., and the loading contact radius was considered
infinitesimal (.about.0). Under these conditions, the calculated
stress was about 563 MPa at the center of the top and bottom
surface. From the results shown in FIG. 7, a value of 0.34 nm/GPa
was determined for the stress coefficient of the R.sub.1 and
R.sub.2 bands (compressive and tensile), which is close to a
previously determined value of 0.38 nm/GPa for uniformly doped
sapphire. The two peaks should have slightly different values to
their stress-dependent shifts, which is a result of the anisotropy
of the sapphire crystal.
[0084] In a chromium doped sapphire window, peak emission lines
react in a predictable fashion in response to the local temperature
and mechanical stress of the sample. Along these lines, an increase
in sample temperature results in a reduction, broadening,
red-shifting, and merging of the bands. Compressive mechanical
stress results in a red-shift of the bands without reduction,
broadening, or merging. Tensile mechanical stress produces such a
shift to the blue.
[0085] An alternative to the apparatus described above and shown in
FIGS. 2 and 3 could include a window support that operable to
support a sapphire window in wind tunnel, where the window could be
subjected to conditions that more closely approach the conditions
that would be encountered in flight. Such an apparatus could still
include elements for delivering exciting radiation and collecting
fluorescence as well as generating heat if necessary.
[0086] The foregoing description of the invention illustrates and
describes the present invention. Additionally, the disclosure shows
and describes only the preferred embodiments of the invention, but
as aforementioned, it is to be understood that the invention is
capable of use in various other combinations, modifications, and
environments and is capable of changes or modifications within the
scope of the inventive concept as expressed herein, commensurate
with the above teachings, and/or the skill or knowledge of the
relevant art. The embodiments described hereinabove are further
intended to explain best modes known of practicing the invention
and to enable others skilled in the art to utilize the invention in
such, or other, embodiments and with the various modifications
required by the particular applications or uses of the invention.
Accordingly, the description is not intended to limit the invention
to the form disclosed herein. Also, it is intended that the
appended claims be construed to include alternative
embodiments.
* * * * *